The present invention relates generally to a technique for adjusting temperature of an object, more particularly to a cooling apparatus for cooling an object such as an optical element in an exposure apparatus that exposes an object, such as a semiconductor wafer and a glass plate for a liquid crystal display (“LCD”), to light. The present invention is suitable, for example, for a cooling apparatus for cooling an object in an exposure apparatus that uses as exposure light ultraviolet (“UV”) light and extreme ultraviolet (“EUV”) light.
Reduction projection exposure apparatus have been conventionally employed which use a projection optical system to project a circuit pattern formed on a mask or a reticle onto a wafer, etc. to transfer the circuit pattern, in manufacturing such fine semiconductor devices as semiconductor memories and logic circuits in photolithography technology.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Recent demands for finer semiconductor devices have promoted a shorter wavelength of ultraviolet light from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has the limit to satisfy the rapidly progressing fine processing of semiconductor devices, and an EUV exposure apparatus using EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet has been developed for efficient transfers of very fine circuit patterns.
In the wavelength range of the EUV light, an attenuation of energy by a gas is very large. Moreover, a carbon compound adheres to the optical element by a photochemical reaction of oxygen and impurities in the gas. Therefore, the exposure is executed in a vacuum environment.
On the other hand, since the EUV exposure apparatus is used to expose circuit patterns of 0.1 μm or smaller and required to meet very high critical dimension accuracy, only a deformation of about 0.1 nm or smaller is permissible on the optical element (in other words, the mirror surface). Deforms a shape of the optical element causes a deterioration of an optical performance, in particular, imaging performance. A mirror used for the EUV exposure apparatus does not reflect all the exposure light, but absorbs the exposure light of 30% or greater. Then, a temperature of the mirror rises gradually, and the surface shape of the mirror deforms. Therefore, it is necessary to cool the optical element. In other words, it is necessary to adjust the temperature of the optical element. However, because the EUV exposure apparatus exposes in the vacuum environment, the temperature adjustment is very difficult compared with the conventional. For example, the surface of the optical element can not be cooled by a convection in the vacuum environment. The temperature adjustment by supplying a cooling medium to a channel formed in the optical element generates a vibration, and a transfer position accuracy is deteriorated.
Then, it is thought to use a radiation (heat radiation) as cooling method of the optical element in the vacuum environment, in a non-contact manner. Concretely, a radiation board provides at a position opposite to a surface except an irradiation area irradiated to the exposure light on the optical element, and the heat is absorbed (radiation cooling) from the optical element through the radiation board. See, for example, Japanese Patent Applications, Publication Nos. 2004-80025 and 2004-29314.
When the radiation cooling is executed through the radiation board provided at the position opposite to the surface except the irradiation area on the optical element, the heat applied by the exposure light to the irradiation area moves to the surface of the optical element opposed to the radiation board, and is recovered. Therefore, a flow of the heat is generated in the optical element, and a temperature distribution is formed. The temperature distribution causes a heat distortion of the optical element and the deterioration of the optical performance. Especially, an exposure energy at unit time is enlarged to improve a throughput of the exposure apparatus, the temperature distribution is enlarged, and the optical performance is remarkably deteriorated.
If the radiation board provides at a position opposite to the irradiation area irradiated to the exposure light on the optical element, the irradiation area can be directly cooled, a heat quantity moved in the optical element decreases, and the temperature distribution generated in the optical element can be decreased. The irradiation area is located on an optical path of the exposure light, and the radiation board can not provide at the position opposite to the irradiation area.